The preparation of glass-ceramic articles exhibiting the capability of being sawed, drilled, punched, or otherwise shaped with hand or machine tools is known to the art. Those articles have contained synthetic micas, commonly termed fluormicas or fluorophlogopites, as the predominant crystal phase. Naturally-occurring micas are usually hydroxyl silicates. The micas developed synthetically have typically contemplated replacing the hydroxyl groups within the crystal lattice of the mica with fluorine. Customarily, the fluormica crystals generated in situ in glass-ceramic articles have been fine-grained and, as such, do not demonstrate the single crystal flexibility frequently exhibited by naturally-occurring micas. However, the synthetic products can display excellent dielectric properties, thermal stability, and mechanical machinability.
The production of fluormica-containing, manually-machinable glass-ceramic bodies was first disclosed in U.S. Pat. No. 3,689,293. The articles described therein consisted essentially, expressed in terms of weight percent on the oxide basis, of about 25-60% SiO.sub.2, 15-35% R.sub.2 O.sub.3, wherein R.sub.2 O.sub.3 consists of 3-15% B.sub.2 O.sub.3 and 5-25% Al.sub.2 O.sub.3, 2-20% R.sub.2 O, wherein R.sub.2 O consists of 0-15% Na.sub.2 O, 0-15% K.sub.2 O, 0-15% Rb.sub.2 O, and 0-20% Cs.sub.2 O, 4-20% F, and 6-25% MgO+Li.sub.2 O, consisting of 4-25% MgO and 0-7% Li.sub.2 O.
X-ray diffraction analyses of the crystals indicated a basic mica structure composed of a fluorophlogopite solid solution, this solid solution being hypothesized to include three components, i.e., normal fluorophlogopite, KMg.sub.3 AlSi.sub.3 O.sub.10 F.sub.2, boron fluorophlogopite, KMg.sub.3 BSi.sub.3 O.sub.10 F.sub.2 and a subpotassic aluminous phlogopite believed to approximate K.sub.0.5 Mg.sub.2 Al.sub.0.83 BSi.sub.3 O.sub.10 F.sub.2.
U.S. Pat. No. 3,756,838 describes the production of glass-ceramic articles wherein an alkali metal-free fluormica comprises the predominant crystal phase. The compositions therefor consist essentially, expressed in terms of weight percent on the oxide basis, of about 30-65% SiO.sub.2, 5-26% Al.sub.2 O.sub.3, 10-35% MgO, 3-15% F, and 3-30% RO, wherein RO consists of 3-30% SrO and 0-25% BaO. Up to several percent individually of a number of metal oxides may optionally be present, but the total amount thereof will not exceed 10% by weight. Those possible additions were selected from the group of As.sub.2 O.sub.3, B.sub.2 O.sub.3, BeO, CaO, Fe.sub.2 O.sub.3, La.sub.2 O.sub.3, MnO, PbO, P.sub.2 O.sub.5, Sb.sub.2 O.sub.3, SnO.sub.2, TiO.sub.2, ZnO, and ZrO.sub.2. K.sub.2 O, Rb.sub.2 O, and Cs.sub.2 O will be avoided because of their ready substitution for BaO and SrO.
The products exhibited the capability of being manually machined and contained fluormica solid solutions varying between about RMg.sub.2.5 AlSi.sub.3 O.sub.10 F.sub.2 and R.sub.0.5 MgAlSi.sub.3 O.sub.10 F.sub.2. The presence of Sr.sup.+2 ions was found to be necessary in the initial batch to stabilize precursor glass formation. Hence, the total substitution of Ba.sup.+2 ions for Sr.sup.+2 was observed to cause the melt to quickly and spontaneously devitrify during cooling to a glass body.
Where less than 5% by weight BaO was present in the composition, the resulting glass-ceramic bodies swelled when contacted with water, leading to the subsequent disintegration thereof. Furthermore, SrO-containing fluormica glass-ceramics and their intermediates with BaO additions demonstrated a great tendency to develop cracks when the parent glass body was heat treated to effect crystallization in situ. Hence, almost invariably the articles developed concentric cracks during heat treatment, the origin of which is not well understood.
U.S. application Ser. No. 349,001, filed concurrently herewith by the present applicant and entitled "Calcium Fluorophlogopite Glass-Ceramics", discloses the preparation of mechanically-machinable, alkali metal-free, glass-ceramic articles wherein the predominant crystal phase is a calcium fluorophlogopite. The base compositions therefor consist essentially, expressed in terms of weight percent on the oxide basis, of about 5-20% CaO, 15-25% MgO, 5-20% Al.sub.2 O.sub.3, 35-60% SiO.sub.2, and 5-15% F which are nucleated with 0.5-3.5% BaO+SrO, consisting of 0-3.5% BaO and 0-2.5% SrO, or, if BaO and/or SrO are absent, then with 8-15% TiO.sub.2.
The alkali metal-free products demonstrate dielectric properties much superior to those glass-ceramic bodies containing "conventional" fluorophlogopites. Moreover, the alkaline earth metal-containing fluormica glass-ceramics have appeared to be mechanically stronger than those bodies wherein alkali metal-containing fluormicas constitute the predominant crystal phase.
Composite articles composed of a substrate of dielectric material, at least one surface of which has an electrically conductive metallic coating thereon, are known to the art for such applications as printed circuit boards. For example, copper clad, epoxy-glass fiber laminates have been widely used for printed circuit boards, and boards utilizing an alumina substrate have been employed where higher temperatures are to be encountered. Because of the inherent high cost of sintered alumina substrates and the difficulty encountered in forming complex shapes thereof, glass and glass-ceramic substrates have also been used in high temperature applications. In the main, the boards utilized commercially have contemplated bonding the metal surface layer to the substrate.
It has been appreciated that a conductive metallic layer integral with the substrate would possess the intrinsic advantages of a very high bonding strength, freedom from concern regarding delamination during soldering or repairing or other surface defect in the bonding of the metal to the substrate, and good control over the depth of the conductive layer. Research has been conducted in the past to develop integral surface films on glass and glass-ceramic articles. This research has been based upon the phenomenon of metal migration to the surface upon heating the article. Illustrations of this research include:
U.S. Pat. No. 3,231,456 describes the production of glass-ceramic articles having integral surface layers of copper, gold, or silver via heat treating base glass compositions in the Li.sub.2 O-MgO-Al.sub.2 O.sub.3 -SiO.sub.2 -P.sub.2 O.sub.5 and Li.sub.2 O-ZnO-SiO.sub.2 -P.sub.2 O.sub.5 systems containing copper, gold, or silver compounds in a reducing atmosphere.
U.S. Pat. No. 3,420,645 is concerned with the manufacture of hollow glass particles from alumino-silicate base compositions and having an integral surface layer of metallic copper. The mechanism involved in the development of the surface layer is expressly stated to be the migration of copper ions in the interior of the glass bodies to the surface as they are heated in a reducing environment.
U.S. Pat. No. 3,464,806 discloses the preparation of glass-ceramic bodies having integral metallic surface layers of copper and/or silver by heat treating in a reducing atmosphere glass bodies having compositions selected from the group of Li.sub.2 O, MgO, Al.sub.2 O.sub.3, and SiO.sub.2, and Li.sub.2 O, MgO, Al.sub.2 O.sub.3, SiO.sub.2, and F which are nucleated with TiO.sub.2 and/or ZrO.sub.2 and contain Ag.sub.2 O and/or Cu.sub.2 O.
U.S. Pat. No. 3,490,887 teaches heat treating in a reducing atmosphere a ferroelectric glass-ceramic body containing cuprous oxide, wherein the internal crystal phase is selected from the group of titanates, niobates, tantalates, and zirconates of Group I and Group II elements and mixtures thereof, to cause the migration of cuprous ions to the surface of the body and the reduction thereof to metallic copper.
U.S. Pat. No. 3,790,360 discusses the production of glass-ceramic bodies having integral surface layers of metallic copper or silver by heat treating in a reducing atmosphere a copper or silver-containing glass having a base composition selected from the group of Li.sub.2 O-Al.sub.2 O.sub.3 -SiO.sub.2, Li.sub.2 O-Al.sub.2 O.sub.3 -MgO-SiO.sub.2, Li.sub.2 O-MgO-CaO-Al.sub.2 O.sub.3 -SiO.sub.2, and MgO-CaO-Al.sub.2 O.sub.3 -SiO.sub.2, which is nucleated with TiO.sub.2 and/or ZrO.sub.2 and/or P.sub.2 O.sub.5 and/or F.
U.S. Pat. No. 3,802,892 involves making glass-ceramic articles having integral copper surface layers by heat treating in a reducing atmosphere base glasses having compositions selected from the group of Na.sub.2 O-K.sub.2 O-Al.sub.2 O.sub.3 -SiO.sub.2 -TiO.sub.2 -CuO, Na.sub.2 O-K.sub.2 O-CaO-Al.sub.2 O.sub.3 -SiO.sub.2 -TiO.sub.2 -CuO, and Al.sub.2 O.sub.3 -SiO.sub.2 -TiO.sub.2 -ZrO.sub.2 -CuO.
U.S. Pat. No. 3,876,407 describes the manufacture of metal-coated glass-ceramic articles from glass articles having base compositions in the Al.sub.2 O.sub.3 -SiO.sub.2 system containing copper and/or silver compounds. The surface of the glass articles is covered with a substance containing copper and/or silver. Thereafter, the coated glass article is heat treated to convert the glass to a glass-ceramic and to cause the copper and/or silver ions therein to migrate to the surface to thereby form a metallic layer of increased thickness.
U.S. Pat. No. 3,892,904 is directed to the preparation of glass-ceramic articles having a metallic coating in localized areas on the surfaces thereof. The method comprises contacting a molten mass of glass which is capable of being converted into a glass-ceramic with another molten mass of a glass which is likewise convertible into a glass-ceramic, but which also contains a copper and/or a silver compound, forming the resulting mass into a glass article, and thereafter heat treating the glass article in a reducing atmosphere to cause the glass to crystallize in situ and to cause the migration of copper and/or silver ions to the surface of the final article in that portion thereof resulting from the glass which initially contained the copper and/or silver compound.
U.S. Pat. No. 4,084,972 describes the production of glass-ceramic articles having a surface layer of alpha-iron crystals. A base glass containing an iron compound is first crystallized in situ in an oxidizing atmosphere to develop a glass-ceramic having an integral surface layer of hematite, following which the glass-ceramic body is fired in a dry reducing atmosphere to convert the hematite to alpha-iron.
U.S. Pat. No. 4,198,466 is concerned with the making of glass-ceramic articles having internal crystallization of beta-spodumene solid solution and an integral surface layer of cobalt-iron, nickel-iron, or cobalt-nickel-iron crystals. Yet again, the surface layer results from the migration of cobalt, nickel, and iron ions to the surface where they are reduced to metals during a heat treatment in a reducing environment.
Although it is apparent from the above illustrations that considerable research has been conducted to produce integral metallic films on the surface of glass-ceramic articles, there is no evidence of any attempt to prepare such products utilizing a machinable glass-ceramic as the substrate body. It requires a complex and multiple-step process to produce metal plated through holes in the conventional printed circuit boards. Moreover, such products are limited to plastic laminates because alumina substrates are too brittle to have holes bored, drilled, punched, or otherwise made therein. Alumina, therefore, is used only for one-sided printed circuitry. In contrast, the use of a machinable glass-ceramic as a substrate material would permit the easy fabrication of articles of complex geometry and would offer the capability of producing holes, the sides of which would be metal plated.
Accordingly, the prinicpal objective of this invention is to provide machinable glass-ceramic bodies having integral, electrically conductive surface films thereon and a method for making them. A practical objective of this invention is to provide printed circuit boards fabricated from machinable glass-ceramic materials having holes therein plated with integral, electrically conductive films.